fork(2)

NAME

fork() — create a new process

SYNOPSIS

#include <unistd.h>

pid_t fork(void);

DESCRIPTION

The fork() system call causes the creation of a new process. The new child process is created with exactly one thread or lightweight process. The new child process contains a replica of the calling thread (if the calling process is multi-threaded) and its entire address space, possibly including the state of mutexes and other resources.

If the calling process is multi-threaded, the child process may only execute async-signal safe functions until one of the exec functions is called. Fork handlers may be installed via pthread_atfork() in order to maintain application invariants across fork() calls (i.e, release resources such as mutexes in the child process).

The child process inherits the following attributes from the parent process:

Real, effective, and saved user IDs.
Real, effective, and saved group IDs.
List of supplementary group IDs (see getgroups(2)).
Process group ID.
Environment.
File descriptors.
Close-on-exec flags (see exec(2)).
Signal handling settings (SIG_DFL, SIG_IGN, address).
Signal mask (see sigprocmask(2)).
Profiling on/off status (see profil(2)).
Command name in the accounting record (see acct(4)).
Nice value (see nice(2)).
All attached shared memory segments (see shmop(2)).
Current working directory.
Root directory (see chroot(2)).
File mode creation mask (see umask(2)).
File size limit (see ulimit(2)).
Real-time priority (see rtprio(2)).
Fine-grained privileges (if installed) (see privileges(5)).
Compartments (if installed and enabled) (see compartments(5)).
The trace flag (see the ttrace(2) TT_PROC_SETTRC request).
Core file settings (see coreadm(2) and coreadm(1M)).

Each of the child's file descriptors shares a common open file description with the corresponding file descriptor of the parent. This implies that changes to the file offset, file access mode, and file status flags of file descriptors in the parent also affect those in the child, and vice-versa.

The child process differs from the parent process in the following ways:

The child process has a unique process ID.
The child process ID does not match any active process group ID.
The child process has a different parent process ID (which is the process ID of the parent process).
The set of signals pending for the child process is initialized to the empty set.
The AFORK flag in the ac_flags component of the accounting record is set in the child process.
Process locks, text locks, and data locks are not inherited by the child (see plock(2)).
All semadj values are cleared (see semop(2)).
The child process's values for tms_utime, tms_stime, tms_cutime, and tms_cstime are set to zero (see times(2)).
The time left until an alarm clock signal is reset to 0 (clearing any pending alarm), and all interval timers are set to 0 (disabled).
Fine-grained privileges are inherited through a calculation to determine which, if any, of the parent process' privileges are inherited by the child.

The vfork() system call can be used to fork processes more quickly than fork(), but has some restrictions. See vfork(2) for details.

If a parent and child process both have a file opened and the parent or child closes the file, the file is still open for the other process.

Security Restrictions

This system call requires the LIMIT privilege in order to exceed the per-user process limit Processes owned by the superuser have this privilege. Processes owned by other users may have this privilege, depending on system configuration. See privileges(5) for more information about privileged access on systems that support fine-grained privileges.

RETURN VALUE

Upon successful completion, fork() returns a value of 0 to the child process and returns the process ID of the child process to the parent process. Otherwise, a value of -1 is returned to the parent process, no child process is created, and errno is set to indicate the error.

The parent and child processes resume execution immediately after the fork() call; they are distinguished by the value returned by fork().

ERRORS

If fork() fails, errno is set to one of the following values.

EAGAIN: The system-imposed limit on the total number of processes under execution would be exceeded.
EAGAIN: The system-imposed limit on the total number of processes under execution by a single user would be exceeded.
ENOMEM: There is insufficient swap space and/or physical memory available in which to create the new process.
EPERM: The PRIV_FORK privilege is not present in the effective set of the calling process.

WARNINGS

Standard I/O streams (see stdio(3S)) are duplicated in the child. Therefore, if fork() is called after a buffered I/O operation without first closing or flushing the associated standard I/O stream (see fclose(3S)), the buffered input or output might be duplicated.

DEPENDENCIES

HP Process Resource Manager

If the optional HP Process Resource Manager (PRM) software is installed and configured, the child process inherits the parent's process resource group ID. See prmconfig(1) for a description of how to configure HP PRM, and prmconf(4) for the definition of process resource group.

AUTHOR

fork() was developed by AT&T, the University of California, Berkeley, and HP.

STANDARDS CONFORMANCE

fork(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1